This is a Laboratory Journal of Organic Chemistry Note for synthesis reaction data obtained in the Organic Chemistry Laboratory — all information was experimentally conducted. This article should be used as a reference when conducting your own synthesis reactions.
The three synthesis reactions covered in this experiment are all substitution reactions. The first synthesis is a reflex reaction leading to yielding of primary alcohol, and involves 2 reactants – NaBr and H2SO4. Since it is bimolecular in nature, it can be easily classified as a Sn2 reaction.
The second synthesis can be carried out easily in the presence of only one reactant, i.e. sodium bromide, therefore, it can be classified as a Sn1 reaction. The third synthesis is carried out in presence of two reactants, zinc chloride (ZnCl) and hydrochloric acid (HCl), which makes it a bimolecular reaction. Therefore, the third synthesis is a Sn2 reaction.
During the reaction of alcohols with a halide of hydrogen, an alkyl halide and water are produced via the process of substitution. The alcohols generally react in this order: Tertiary > Secondary > Primary methyl. The hydrogen halides react in this order: HI > HBr > HCl. HF is mostly unreactive. An acid acts as a catalyst to the reaction. Alcohols have a tendancy to react with the strongly acidic halides HCl, HBr, and HI, but they do not produce any reaction with the non-acidic NaCl, NaI, or NaBr. Primary and secondary alcohols, when allowed to react with a mixture of sodium halide and sulphuric acid, convert to alkyl chlorides and bromides.
Certain classes of alcohols react by a mechanism, which leads to the formation of a carbocation, in a SN1 reaction, with the substrate as protonated alcohol.
The SN1 mechanism begins with protonating an alcohol to produce an oxonium ion. About the oxonium ion, even though it is produced by an alcohol, can also be viewed as a Lewis acid-base complex between a cation (R+) and water. This process of alcohol protonation has the capability of converting a poor leaving group (OH-) to a good leaving group, making the dissociation much more feasible. The reaction of conversion of an alcohol to an alkyl halide is carried out in the process of acid and halide ions, and at a controlled temperature. Halide ions are known to be excellent nucleophiles. Their presence in high concentrations makes most of the carbocations to react with the electron pairs of the halide ions to produce a more stable species, alkyl halide. This whole process is called Sn1 reaction.
The protonation of alcohol hydroxyl group with an acid converts it to a good leaving group. However, hydrohalic acids can be replaced by other strong Lewis acids. Since a chloride ion is a weak nucleophile, HCl does not produce any reaction with either primary or secondary alcohols, unless there is an addition of ZnCl or a similar Lewis acid to the mixture. Zinc chloride associates with the unshared pair of electrons of oxygen atom to form a complex with alcohol. This leads to the enhancement of the hydroxyl’s leaving group potential and thus makes it easy for the chloride to displace it.
The second type of substitution reaction is Sn2 reaction. It is named such because this process involves two molecules in the state of transition. The leaving group departs at the same time when the nucleophile attacks. Thus, it leads to the inversion of stereo center.
The main concept to be understood while discussing an SN2 reaction is that of a “backside attack”. In this process, a nucleophile attacks on alkyl halide at 180°. As the nucleophile bonds with the C, it leads to breaking of the bond C-(leaving group). At this state of transition in the reaction, partial C-nucleophile bonds co-exist with the partial C-leaving group bonds.
These reactions should be undertaken in proper conditions and the weights of each particular must be measured properly. Failure to do so can result in a side reaction or a reverse reaction, which can affect the final product and yield. Heat should be properly maintained and while heating, care should be taken that the final product is not burnt. Reflux time and final washing can also change the yield, hence proper care must be taken.
Yield in the process depends on the SN1 and SN2 reactions, which occur between alcohol and halide. The yield was lesser in the experiments because of production of side products and water, which reduces the reactions to produce the final product. If the rate of reaction is faster than expected, it can also lead to a lower yield of the final product. Conversion ratio in final product depends on the reaction time as well as the materials used in the process. Side products like aqueous layers can also lower the output of the yield.